netdev - Re: [PATCH net-next 2/3] rds: Enable RDS IPv6 support

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Date:   Wed, 27 Jun 2018 06:29:23 -0400
From:   Sowmini Varadhan <sowmini.varadhan@...cle.com>
To:     Ka-Cheong Poon <ka-cheong.poon@...cle.com>
Cc:     netdev@...r.kernel.org, santosh.shilimkar@...cle.com,
        davem@...emloft.net, rds-devel@....oracle.com
Subject: Re: [PATCH net-next 2/3] rds: Enable RDS IPv6 support

On (06/27/18 18:07), Ka-Cheong Poon wrote:
> 
> There is a reason for that.  It is the way folks expect
> how IPv6 addresses are being used.

have you tried "traceoute6 -s abc::2 fe80::2" on linux?

> It is not just forwarding.  The simple case is that one
> picks a global address in a different link and then
> use it to send to a link local address in another link.

This is actually not any different than ipv4's strong/weak ES model.

Global addresses are supposed to be globally routable. For your
above example, if yuu do that, it is assumed that your routing
table has been set up suitably.

To state what may be well-known:
This does not work for link-locals, becuase, as the name 
suggests, those are local to the link and you may have the same
link-local on multiple links

> This does not work.  And the RDS connection created will
> be stuck forever.  

that is a different problem in the RDS implementation (that
it does not backoff and timeout a failing reconnect)

As you can see from the traceroute6 example, global <-> link-local 
is supported for udp (and probably also tcp sockets, I have not checked
that case)

> I don't expect RDS apps will want to use link local address
> in the first place.  In fact, most normal network apps don't.
   :
> Do you know of any IPv4 RDS app which uses IPv4 link local
> address?  In fact, IPv4 link local address is explicitly
> disallowed for active active bonding.

Are we talking about "why this ok for my particular use
of link-local, so I can slide my patch forward" or, 
"why this is correct IPv6 behavior"?

> Can you explain why DNS name resolution will return an IPv6
> link local address?  I'm surprised if it actually does.

It depends on how you set up your DNS.

It seems like this is all about "I dont want to deal with this
now", so I dont want to continue this discussion which is really
going nowhere.

Thanks

--Sowmini